1. Field of the invention
The present invention relates to a film deposition apparatus, and more specifically to an improved film deposition apparatus suitable in particular for depositing an oxide superconducting thin film or a stacked multi-layer structure including one or more oxide superconducting thin films
2. Description of related art
Oxide superconductors have been considered to have a critical temperature higher than that of a conventional metal type superconductor, and therefore, to have high possibility of practical use. For example, it has been reported that an Y-Ba-Cu-O compound oxide superconductor material has the critical temperature not less than 80K, and a Bi-Sr-Ca-Cu-O compound oxide superconductor material and a Tl-Ba-Ca-Cu-O compound oxide superconductor material has the critical temperature not less than 100K. The oxide superconductor has a crystalline anisotropy in superconductivity characteristics, and in particular, its critical current density is maximum in a direction perpendicular to a c-axis of crystal lattice. Therefore, when the oxide superconductor is used, attention should be paid to the crystalline orientation.
In the case of applying the oxide superconductor to superconducting electronics including superconducting devices and superconducting integrated circuits, the oxide superconductor has to be used in the form of a thin film. When the oxide superconductor is in the form of a thin film, the above mentioned crystalline anisotropy of superconductivity characteristics becomes more remarkable. In addition, to realize high performance superconducting devices and superconducting integrated circuits, an oxide superconducting thin film having an excellent crystallinity is required.
In order to form the oxide superconducting thin film having an excellent crystallinity, various types of apparatuses such as sputtering, laser ablation and the others have conventionally been used in many cases. However, in the case that a precise film thickness control and a continuous film deposition are required as when an active region of a superconducting field effect transistor type device is formed, it has now been considered that a molecular beam epitaxy (abbreviated "MBE" hereinafter) is optimum. The MBE process enables not only to precisely control a film thickness, by a layer-by-layer deposition, under a ultra-high vacuum, but also to continuously deposit a plurality of oxide superconducting thin films. Therefore, it is possible to form a less disturbed superconducting current path formed of an oxide superconducting thin film, without a large density of unnecessary energy levels, and also to form a sharp junction.
For example, one typical conventional MBE apparatus basically includes a vacuum chamber provided with a main evacuating apparatus, at least one Knudsen's cell (called "K cell" hereinafter) provided at a bottom of the vacuum chamber and for accommodating therein an evaporation source, and a sample holder provided at a top of the vacuum chamber for holding a substrate on which a film is to be deposited. The sample holder is associated with a heater for heating the substrate. In addition, the vacuum chamber is also provided with a port for exchanging a sample, a liquid nitrogen shroud for forming a cold trap around the evaporation source, and a reflecting high energy electron diffraction (called "RHEED" hereinafter) device for observing a thin film roughness during deposition. In front of the substrate held by the sample holder, a shutter is located for controlling the deposition time during the deposition process. In some cases, an electron beam gun (called an "EB gun" sometimes hereinafter) can be provided in place of the K cell. The K cell and the EB gun is provided with an openable shutter.
When an oxide superconducting thin film is deposited in the above mentioned MBE apparatus, a gas supplying apparatus is provided so as to introduce an oxidizing gas such as O.sub.3, NO.sub.2, N.sub.2 O, etc. in the proximity of the substrate held by the sample holder. In other words, in order to oxidize metal molecular beams incoming from the evaporation source, it is necessary to supply oxygen in the course of the film deposition so as to maintain an oxygen-enriched atmosphere. Therefore, there is provided the gas supplying device which is not used in conventional MBE apparatuses, so that the oxidizing gas such as O.sub.3 is supplied in the proximity of the substrate in the course of the film deposition, whereby a film is deposited while an active oxygen is supplied onto the front surface of the substrate.
However, in the case of manufacturing a superconducting device such as the superconducting field effect transistor type device mentioned hereinbefore and the other type devices, it is required to form a thin film other than the oxide superconducting thin film. The thin film can be exemplified by a thin film of SrTiO.sub.3, Si.sub.3 N.sub.4, etc., used for constituting various insulating films, and by a noble metal film of Au, Pt, etc., used as an electrode. However, it is preferred that these thin films are deposited by a sputtering process, a laser ablation process and a .vacuum evaporation process.
On the other hand, if the oxide superconducting thin film is exposed to air, an exposed surface of the oxide superconducting thin film is deteriorated, so that the superconductivity and the crystallinity are destroyed in some cases. In order to avoid this problem, when a different thin film is deposited on the oxide superconducting thin film, a continuous film deposition process has been proposed in which, after the oxide superconducting thin film has been deposited, a different thin film is deposited on the oxide superconducting thin film without being exposed to air.
For example, the continuous film deposition process can be realized by a system in which an MBE apparatus, a sputtering apparatus and a laser ablation apparatus are coupled to one another by a vacuum tunnel, which is in turn connected through an evacuating tube to a vacuum system, so that the MBE apparatus, the sputtering apparatus and the laser ablation apparatus are simultaneously evacuated. The MBE apparatus is additionally provided with another vacuum pump, so that the MBE apparatus can be evacuated to a degree of vacuum higher than that of the sputtering apparatus and the laser ablation apparatus.
With the above mentioned system, a substrate to be deposited can be moved through the vacuum tunnel to any one of the MBE apparatus, the sputtering apparatus and the laser ablation apparatus without being exposed to air.
Furthermore, a load lock type MBE apparatus has been proposed, which mainly includes a vacuum chamber provided with a main evacuating device, a plurality of K cells provided at a bottom of the vacuum chamber and each for accommodating therein an evaporation source, and a sample holder provided at a top of the vacuum chamber for holding a substrate on which a film is to be deposited. In addition, the vacuum chamber is also provided with a liquid nitrogen shroud for forming a cold trap around the evaporation source, and a RHEED device for observing a thin film being deposited. Furthermore, the vacuum chamber is associated with a sample introducing chamber, which is in turn coupled to the vacuum chamber through a gate valve and provided with a sample exchanging port and an auxiliary evacuating device. The gate valve can hermetically shut off communication between the vacuum chamber and the sample introducing chamber.
With this arrangement, when the gate valve is closed so as to maintain a vacuum condition of the vacuum chamber, a substrate is introduced into the sample introducing chamber. After the sample introducing chamber is closed and evacuated by the auxiliary evacuating device until a pressure of the sample introducing chamber is rendered substantially equal to that of the vacuum chamber. Thereafter, the gate valve is opened so that the substrate is moved from the sample introducing chamber to the vacuum chamber. Therefore, the substrate can be introduced into the vacuum chamber and the substrate can be exchanged without breaking the vacuum condition of the vacuum chamber. Accordingly, it is possible to shorten a start-up time of the film deposition processing, and also, contamination of the evaporation source can be effectively prevented.
Here, reviewing the sample holder of the conventional MBE apparatus, the sample holder basically comprises a circular disk member having a front surface integrally provided with a heater and a guide member for guiding and holding a substrate holder. The circular disk member is supported at its rear surface by a tip end of first supporting rods. At the rear surface of the circular disk member, a radiator is supported by second supporting rods. In addition, a pair of power supply wires for supplying an electric power to the heater extend from the rear surface of the circular disk member through the circular disk member to the heater.
Furthermore, the guide member is provided with a plurality of resilient bent members for holding the substrate holder. The substrate holder is in the form of a cap, and a sample or substrate is fixed on an outer surface of a bottom of the cap-shaped substrate holder. An outer surface of an edge portion of a cylindrical section of the cap-shaped substrate holder is provided with a groove. The substrate holder is transported by a magnet coupling transfer rod and is fitted to the tip end of the sample holder by causing the resilient bent members of the guide member to be fitted into the outer groove of the cap-shaped substrate holder. In this condition, the substrate holder is so sized that a predetermined gap is maintained between the heater and an inner surface of the substrate holder when the resilient bent members of the guide member are fitted into the outer groove of the cap-shaped substrate holder. Thus, the gap maintained between the heater and an inner surface of the substrate holder ensures that the fitting and removal of the substrate holder can be smoothly performed.
The film deposition using the above mentioned MBE apparatus is performed in the procedure in which the vacuum chamber is evacuated to a high degree of vacuum (ultra low pressure), and then, the substrate fixed on the substrate holder is heated to a predetermined temperature, and thereafter, the evaporation source is properly heated so that a molecular beam is generated and a thin film is deposited on the substrate.
In the film deposition in accordance with the MBE process, generally, it is necessary to maintain at least the proximity or surrounding of the evaporation sources at a high vacuum of not greater than 10.sup.-6 Torr. On the other hand, if a partial pressure of oxygen on the order of several tens mTorr is not ensured in the neighborhood of the film deposition surface of the substrate, an atmosphere necessary for oxidation cannot be realized. Accordingly, it is necessary to form a sufficient pressure difference between the proximity of the evaporation sources and the neighborhood of the substrate in the single vacuum chamber. However, a satisfactory pressure difference could not be realized.
In addition, the oxide superconducting thin film is transformed from a tetragonal system to an orthorhombic system, and this transformation occurs about 400.degree. C. At this stage, the neighborhood of the deposited film is required to be an oxygen pressure of substantially normal pressures. This can be realized by introducing a large amount of oxygen gas into the vacuum chamber after the film deposition has been completed in the conventional MBE apparatus. However, the evaporation source is oxidized by the introduced oxygen gas and becomes unable to be used again.
Furthermore, if the oxygen gas is introduced in the vacuum chamber to the normal pressures, in order to perform a next film deposition processing, it is necessary to evacuate the vacuum chamber to a ultra-high vacuum, again. Since the evacuation to the vacuum pressure needs a long time, an available time becomes remarkably short.
As mentioned hereinbefore, when a film other than the oxide superconducting thin film is continuously deposited on the oxide superconducting thin film by means of a method other than the MBE process as in the case that the superconducting field effect transistor type device is manufactured, there has been required .the system in which the MBE apparatus, the sputtering apparatus and the laser ablation apparatus are coupled to one another. However, this system is realized by coupling the MBE apparatus, the sputtering apparatus and the laser ablation apparatus independently of each other, and therefore, the system is very expensive. In addition, in order to realize a continuous film deposition process, the substrate must be transported from one apparatus to another, and therefore, productivity is low.
In the conventional MBE apparatus, on the other hand, the substrate temperature is limited to about 700.degree. C. at maximum, since the temperature of the deposition surface of the substrate cannot exceed over 700.degree. C. even if the input power to the heater is increased in attempting to deposit the film at a higher substrate temperature. Because of this, it has been difficult to maintain the substrate temperature suitable for deposition of the oxide superconducting thin film.